It is known to use oxygen as the oxidizing agent instead of air in boilers for producing energy.
The advantage of using oxygen as the oxidizing agent is the reduction or elimination of the nitrogen ballast diluting the carbon dioxide contained in the flue gases and coming from the nitrogen contained in the combustion air, and also the significant reduction in the dimensions of the equipment needed.
The application of this principle to a circulating fluidized bed boiler is disclosed in U.S. Pat. No. 6,505,567. According to that document, a steam generator or circulating fluidized bed boiler furnace includes means for introducing substantially pure oxygen into the generator.
The technical advantage of a circulating fluidized bed is that it enables heat to be extracted from the loop of circulating solids and it enables a low combustion temperature to be conserved that is decoupled from the oxygen content of the oxidizing agent. That technique is thus particularly attractive, and makes it possible to maximize the oxygen fraction in the oxidizing agent and to minimize the size of the boiler, which size depends directly on the rate at which flue gases are produced.
In that technique, the flue gases emitted by the combustion equipment are constituted mainly by carbon dioxide and by water vapor.
Nevertheless, such flue gases also contain a small fraction of non-condensible gas, including oxygen. This oxygen content typically lies in the range 3% to 5% relative to dry gas and cannot be reduced below a minimum threshold of about 2.5%, which is imposed by the quality of combustion and by safety constraints.
The quality of combustion is represented by the CO content in the flue gases produced and by the unburnt solids contained in the combustion by-products.
Unfortunately, the maximum oxygen content that can be accommodated in the specifications for CO2 treatment systems, and above all in transport networks, can be of the order of 100 parts per million by volume (ppm vol), with this requirement being due mainly to the risks of corrosion to equipment conveying CO2, which risk is made worse by the presence of water vapor. For carbon steel pipelines, a mixture of gases CO2, SO2, H2O, and O2 can be incompatible, depending on its proportions, with satisfactory long-term mechanical performance.
It is thus essential to reduce significantly or even to eliminate the oxygen content from the flue gases that are given off, before they penetrate into the CO2 treatment station.
It is known to carry out this reduction in oxygen content by additional catalytic treatments that are complex and that lead to large investment and operating costs.
Furthermore, European patent No. 0 497 418 discloses a method of burning coal in an air circulating fluidized bed boiler that includes a zone in which coal is mixed with hot solid materials coming from the combustion chamber, whereby, at the outlet from a separator for separating gas and solid particles, carbonization gas (also known as pyrolysis gas) produced in said mixing zone is drawn off and added to the combustion gas containing oxygen and leaving the separator, the carbonization gas being burned in such a manner as to raise the temperature to a value in the range 900° C. to 1200° C. in order to reduce nitrous oxide.
The invention relates to a circulating fluidized bed device in which combustion is performed by means of a mixture of oxygen and carbon dioxide capable of containing 21% to 100% oxygen.
The invention proposes adapting this method of using combustible gas, more particularly pyrolysis gas, that is known for an air-fired furnace, to an oxygen-fired furnace for the purpose of eliminating residual oxygen from the combustion gas.